Steel sheet for porcelain enamel, and method of manufacturing same

ABSTRACT

A steel sheet for porcelain enamel according to an embodiment of the present invention comprises 0.0005 to 0.0030 wt % of C, 0.15 to 0.55 wt % of Mn, to 0.03 wt % of S, 0.0001 to 0.002 wt % of Al, 0.001 to 0.020 wt % of P, 0.001 to 0.030 wt % of S, 0.02 to 0.06 wt % of Cu, 0.005 to 0.012 wt % of N, 0.05 to 0.20 wt % of Cr, and 0.03 to 0.06 wt % of O, with the balance being Fe and inevitable impurities. A cold-rolled steel sheet for porcelain enamel according to an embodiment of the present invention comprises an oxide layer that extends inward from the surface, wherein the thickness of the oxide layer is 0.006-0.030 μm.

TECHNICAL FIELD

An exemplary embodiment of the present invention relates to a steel sheet for porcelain enamel and a method of manufacturing the same. More particularly, an exemplary embodiment of the present invention relates to a continuous annealing type steel sheet for porcelain enamel having excellent fish scale resistance and enamel adhesiveness, and having excellent yield strength even after enamel treatment, and a method of manufacturing the same.

BACKGROUND ART

An enamel steel sheet is a kind of surface-treated product having corrosion resistance, weather resistance, heat resistance, and the like which are improved by applying a vitreous glaze on a base steel sheet such as a hot rolled steel sheet or cold rolled steel sheet and then performing firing at a high temperature. The enamel steel sheet as such is being used as materials for building exteriors, home appliances, tableware, and various industrial uses.

As a steel sheet for porcelain enamel, a rimmed steel has been used since the past, but in recent years, as a continuous casting method has been actively used in terms of productivity improvement, most of the materials are subjected to continuous casting. In addition, in the manufacture of steel materials, fish scale defects which are one of the most fatal defects of the enamel steel sheet are representative enamel defects which occur when hydrogen dissolved in a steel during the manufacturing process of an enamel product is supersaturated in the steel in a cooling process after firing and then removes an enamel layer in the form of fish scales while being released to the surface of the steel. When the fish scale defects as such occur, the value of the enamel product is significantly deteriorated, for example, rust occurs intensively on the defective area, and thus, the occurrence of the fish scale needs to be suppressed. In order to prevent the fish scale defects, a position (site) to hold hydrogen dissolved in a steel needs to be formed in a large amount inside the steel. Thus, in order to prevent the fish scale defects which deteriorate enamel properties or improve aging properties, an open coil annealing (OCA) method which is a kind of a batch annealing method may be used, but in this case, productivity declines by a long-term heat treatment to increase manufacturing costs and cause a large difference in quality. In addition, since the open coil annealing method is difficult to control a decarburization amount and causes too much decarburization, when a carbon amount in a steel is too small, the crystal grain boundary of a steel plate is softened to cause cracks such as brittle fracture during product molding. In order to overcome productivity deterioration and a manufacturing cost increase problem, a steel sheet for porcelain enamel which has been recently developed actively uses a continuous annealing process, in which as a hydrogen storage source, precipitates of titanium and the like, inclusions using a non-deoxidized steel, and the like are mainly used. However, this case also acts as a factor of various quality problems such as a high surface defect incidence rate due to a large amount of carbonitride-forming elements added or non-deoxidized compounds and deteriorated threading due to a recrystallization temperature rise, and a decrease in productivity and an increase in cost.

That is, since to an enamel steel sheet using a titanium (Ti)-based precipitate, a large amount of titanium is added for suppressing a hydrogen reaction which is a cause of fish scale, nozzle clogging by titanium nitride (TiN) and inclusion often occurs in a continuous casting step of a steelmaking process, which is a direct cause of poor workability and production load. In addition, TiN incorporated into a molten steel causes blister defects which are present on the upper portion of a steel sheet and are representative air bubble defects, and titanium added in a large amount is also a cause to impair the adhesiveness between a steel sheet and a glaze layer.

In addition, since most of the enamel steel is used as a material of a structural member, strengthening competitiveness through weight reduction of used materials is pursued by increasing the strength of the material. To this end, a yield strength after a firing heat treatment for drying a glaze in an enamel process needs to be secured.

DISCLOSURE Technical Problem

An exemplary embodiment of the present invention attempts to provide a steel sheet for porcelain enamel and a method of manufacturing the same. More specifically, an exemplary embodiment of the present invention attempts to provide a continuous annealing type steel sheet for porcelain enamel for processing which does not cause bubble defect occurrence after treating enamel and has excellent enamel adhesiveness and fish scale resistance, and a method of manufacturing the same.

Technical Solution

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention includes, by weight: 0.0005 to 0.0030% of C, 0.15 to 0.55% of Mn, 0.001 to 0.03% of Si, 0.0001 to 0.002% of Al, 0.001 to of P, 0.001 to 0.030% of S, 0.02 to 0.06% of Cu, 0.005 to 0.012% of N, to 0.20% of Cr, and 0.03 to 0.06% of O, with a balance of Fe and inevitable impurities. The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may include an oxide layer in a direction from the surface to the inside and the oxide layer may have a thickness of 0.006 to 0.030 μm. The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may satisfy the following Equation 1:

3.05≤[Cu]/[P]≤5.10  [Equation 1]

-   -   wherein [Cu] and [P] are contents (wt %) of Cu and P,         respectively. The steel sheet for porcelain enamel according to         an exemplary embodiment of the present invention may satisfy the         following Equation 2:

0.032≤([Cr]/52+[Mn]/32)×([N]/14)/([C]/12)≤0.091

-   -   wherein [Cr], [Mn], [N], and [C] are contents (wt %) of Cr, Mn,         N, and C, respectively. One or more of 0.001 wt % or less of Ti,         0.001 wt % or less of Nb, wt % or less of Ni, 0.001 wt % or less         of V, and 0.02 wt % or less of Mo may be further included. The         steel sheet for porcelain enamel according to an exemplary         embodiment of the present invention may satisfy the following         Equation 3:

0.45≤(R _(a)×50×S _(e))/P _(c)≤0.99  [Equation 3]

-   -   wherein P_(c) is the number of surface irregularities per unit         centimeter (cm), R_(a) is an average roughness value (μm), and         S_(e) is a temper reduction rate (%).

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may have a yield strength of 220 MPa or more after an enamel firing heat treatment.

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may have an enamel adhesiveness of 95% or more.

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may have a hydrogen permeation ratio of 600 sec/mm 2 or more.

A method of manufacturing a steel sheet for porcelain enamel according to an exemplary embodiment of the present invention includes: hot rolling a slab including, by weight: 0.0005 to 0.0030% of C, 0.15 to 0.55% of Mn, 0.001 to 0.03% of Si, 0.0001 to 0.002% of Al, 0.001 to 0.020% of P, 0.001 to 0.030% of S, 0.02 to 0.06% of Cu, 0.005 to 0.012% of N, 0.05 to 0.20% of Cr, and 0.03 to 0.06% of O, with a balance of Fe and inevitable impurities to manufacture a hot rolled steel sheet; cold rolling the hot rolled steel sheet to manufacture a cold rolled steel sheet; annealing the cold rolled steel sheet; and temper rolling the annealed cold rolled steel sheet.

The annealing may be performed at a temperature of 760 to 840° C. for 10 to 90 seconds.

In the manufacturing of a hot rolled steel sheet, a finish hot rolling temperature may be 910 to 970° C.

In the manufacturing of a hot rolled steel sheet, a winding temperature may be 580 to 720° C.

In the manufacturing of a cold rolled steel sheet, a reduction rate may be 60 to 90%. The temper rolling may be rolling at a reduction rate of 0.4 to 2.0%.

After the temper rolling, enamel firing the temper rolled steel sheet at a temperature of 780 to 850° C. may be further included.

Advantageous Effects

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention has excellent fish scale resistance and enamel adhesiveness.

In addition, the steel sheet for porcelain enamel according to an exemplary embodiment of the present invention optimizes surface roughness properties in the heat treating and temper rolling in a continuous annealing furnace after cold rolling, thereby maintaining adhesiveness high.

In addition, the steel sheet for porcelain enamel according to an exemplary embodiment of the present invention forms a Mn—Cr-based precipitate at a high temperature which is used as a hydrogen storage source, thereby preventing fish scale defects caused by hydrogen.

In addition, the steel sheet for porcelain enamel according to an exemplary embodiment of the present invention suppresses crystal grain growth by residual nitrogen on a surface layer in a steel sheet during enamel firing, thereby securing stable material properties even after firing at a high temperature.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a cross-section of a steel sheet for porcelain enamel according to an exemplary embodiment of the present invention.

FIG. 2 is the analysis results of glow discharge spectroscopy (GDS) by depth of a steel sheet for porcelain enamel according to Inventive Example 5.

MODE FOR INVENTION

In the present specification, the terms such as first, second, and third are used for describing various parts, components, areas, layers, and/or sections, but are not limited thereto. These terms are used only for distinguishing one part, component, area, layer, or section from other parts, components, areas, layers, or sections. Therefore, a first component, part, area, layer, or section described below may be mentioned as a second component, part, area, layer, or section without departing from the scope of the present invention.

In the present specification, unless explicitly described to the contrary, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of stated elements but not the exclusion of any other elements.

In the present specification, the terminology used is only for mentioning a certain example, and is not intended to limit the present invention. Singular forms used herein also include plural forms unless otherwise stated clearly to the contrary.

The meaning of “comprising” used in the specification is embodying certain characteristics, regions, integers, steps, operations, elements, and/or components, but is not excluding the presence or addition of other characteristics, regions, integers, steps, operations, elements, and/or components.

In the present specification, the term “combination thereof” included in the Markush format refers to a mixture or combination of one or more selected from the group consisting of the constituent elements described in the Markush format, and refers to inclusion of one or more selected from the group consisting of the constituent elements.

In the present specification, when it is mentioned that a part is “on” or “above” the other part, it means that the part is directly on or above the other part or another part may be interposed therebetween. In contrast, when it is mentioned that a part is “directly on” the other part, it means that nothing is interposed therebetween.

Though not defined otherwise, all terms including technical terms and scientific terms used herein have the same meaning as commonly understood by a person with ordinary skill in the art to which the present invention pertain. Terms defined in commonly used dictionaries are further interpreted as having a meaning consistent with the related technical literatures and the currently disclosed description, and unless otherwise defined, they are not interpreted as having an ideal or very formal meaning.

In addition, unless particularly mentioned, % refers to wt % or mass %, and 1 ppm is 0.0001 wt %.

In an exemplary embodiment of the present invention, the meaning of further including an additional element is including iron (Fe) in a substitute amount for an additional amount of an additional element.

Hereinafter, an exemplary embodiment of the present invention will be described in detail so that a person with ordinary skill in the art to which the present invention pertains can easily carry out the invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

A steel sheet for porcelain enamel according to an exemplary embodiment of the present invention includes, by weight: 0.0005 to 0.0030% of C, 0.15 to 0.55% of Mn, 0.001 to 0.03% of Si, 0.0001 to 0.002% of Al, 0.001 to 0.020% of P, 0.001 to 0.030% of S, 0.02 to 0.06% of Cu, 0.005 to 0.012% of N, 0.05 to 0.20% of Cr, and 0.03 to 0.06% of O, with a balance of Fe and inevitable impurities.

First, the reason for defining the components of the steel sheet will be described. The contents of the elements in the final steel sheet may have a concentration gradient in the thickness direction, and the content of the element described later represents an average of the contents in the entire steel sheet 100 including an oxide layer 20.

C: 0.0005 to 0.0030 wt %

When carbon (C) is added too much, an amount of carbon dissolved in a steel is increased to interfere with the development of texture after cold rolling-annealing, which may deteriorate processability. In addition, bubbling defects may be caused by enamel layer bubbling after treating porcelain enamel. However, when C is added too little, the structure grows so that a target yield strength may not be secured after firing, and a fraction of the precipitate acting as a hydrogen storage source is lowered to cause vulnerability to fish scale defects. More specifically, carbon may be included at 0.0010 to 0.0028 wt %.

Mn: 0.15 to 0.55 wt %

Manganese (Mn) is a representative solid solution strengthening element, and precipitates sulfur dissolved in a steel into a manganese sulfide (MnS) form to prevent hot shortness and create carbide precipitation. When Mn is added too little, it is difficult to obtain the effects described above. However, when the content of Mn is too high, moldability becomes poor and an Ara transformation temperature is lowered to cause transformation during enamel firing, resulting in deformation. Therefore, Mn may be included at 0.15 to 0.55 wt %. More specifically, Mn may be included at 0.20 to 0.55 wt %.

Si: 0.001 to 0.03 wt %

Silicon (Si) is an element which promotes formation of carbides acting as a solid solution strengthening and hydrogen storage source. When Si is added too little, it is difficult to sufficiently obtain the effects described above. However, when Si is added too much, a high-concentration oxide film is formed on the surface of a steel sheet to deteriorate enamel adhesiveness. Therefore, Si may be included at 0.001 to 0.030 wt. More specifically, Si may be included at 0.005 to 0.025 wt %.

Al: 0.0001 to 0.002 wt %

Aluminum (Al) is an element which is used as a strong deoxidizer to remove oxygen in a molten steel and fixes solubilized nitrogen. Since it is necessary to use precipitates and inclusions in a steel as a hydrogen storage source, possible deoxidation may be suppressed. Therefore, the upper limit of Al may be limited to 0.0020 wt %. Since it is preferred to contain Al as little as possible, the lower limit may be limited to 0.0001 wt %. More specifically, Al may be included at 0.0005 to 0.0015 wt %.

P: 0.001 to 0.02 wt %

Phosphorus (P) is a representative material strengthening element. When P is added too little, it is difficult to sufficiently obtain the effects described above. However, when P is added too much, a P segregation layer is formed inside a steel to deteriorate moldability and deteriorate the pickling of a steel to adversely affect enamel adhesiveness. Therefore, P may be included in a range of 0.001 to 0.020 wt %. More specifically, P may be included at 0.005 to 0.015 wt %.

S: 0.001 to 0.030 wt %

Sulfur (S) is an element which is bonded to manganese to cause hot shortness. When S is added too little, weldability may be deteriorated. When S is added too much, ductility is greatly deteriorated to deteriorate processability and manganese sulfide is over-precipitated to adversely affect the fish scaling of a product. Therefore, S may be included at 0.001 to 0.030 wt %. More specifically, S may be included at 0.005 to 0.025 wt %.

Cu: 0.020 to 0.060 wt %

Copper (Cu) is an element added for solid solution strengthening and enamel adhesiveness improvement. When Cu is added too little, the effects described above may not be appropriately obtained. When Cu is added too much, a pickling speed is lowered in a pickling step which is an enamel pretreatment process, so that appropriate roughness properties on the surface of a steel sheet may not be obtained to lower adhesiveness. Therefore, Cu may be included at to 0.060 wt %. More specifically, Cu may be included at 0.025 to 0.055 wt %.

N: 0.005 to 0.012 wt %

Nitrogen (N) is a representative hardening element and is added for obtaining a target yield strength after enamel firing. When N is included too little, the yield strength may be deteriorated after enamel firing. When N is included too much, moldability is deteriorated and surface defects such as bubble defects may occur in an enamel treatment process. Therefore, N may be included at 0.0050 to 0.0120 wt %. More specifically, N may be included at 0.0075 to 0.0110 wt %.

Cr: 0.05 to 0.20 wt %

Chromium (Cr) is an element which forms precipitates and inclusions in a steel, thereby increasing strength and improving fish scale resistance. When Cr is added too little, the effects described above may not be appropriately obtained. When Cr is added too much, it is concentrated on the surface to deteriorate enamel adhesiveness and may act as a cost increase factor by addition of expensive ferroalloy. Therefore, Cr may be included at 0.050 to 0.200 wt %. More specifically, Cr may be included at 0.075 to 0.190 wt %.

0.03 to 0.06 wt %

Oxygen (O) is an element which is essential for forming an oxide in a steel, and the oxides act as an efficient hydrogen storage source to improve fish scale resistance. When O is included too little, the effects described above may not be appropriately obtained. When O is included too much, occurrence of melting loss of refractories in a steel sheet manufacturing step, and occurrence of surface defects such as backline may be increased on the surface of a steel sheet. Therefore, O may be included at 0.0300 to 0.0600 wt %.

Regarding the manufacturing process to be described later, an oxide layer may be formed in an annealing process. However, since the thickness of the oxide layer 20 is too small as compared with the entire steel sheet 100, there is substantially no change in an oxygen amount in the entire steel sheet 100. Oxygen is included at 5 wt % or more in the oxide layer 20. More specifically, O may be included at 10 to 50 wt % in the oxide layer 20. The content of oxygen in the oxide layer 20 refers to an average content in the oxide layer 20.

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may satisfy the following Equation 1:

3.05≤[Cu]/[P]≤5.10  [Equation 1]

-   -   wherein [Cu] and [P] are contents (wt %) of Cu and P,         respectively. When the value of Equation 1 is too small,         appropriate surface characteristics are not secured in a         pretreatment step, and thus, a wedge effect may be decreased to         deteriorate enamel adhesiveness. In contrast, when the value of         Equation 1 is too large, the surface roughness properties         disappear so that an enamel glaze layer flows down, and a gas         inflow is increased toward a surface portion so that enamel         surface defects such as bubble defects often occur which may act         as a factor to deteriorate reliability of a product. Thus, for         securing enamel adhesiveness and suppressing surface bubble         defects, the value of Equation 1 may be defined as 3.05 to 5.10.         More specifically, the value of Equation 1 may be 3.20 to 5.00.

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may satisfy the following Equation 2:

0.032≤([Cr]/52+[Mn]/32)×([N]/14)/([C]/12)≤0.091

-   -   wherein [Cr], [Mn], [N], and [C] are contents (wt %) of Cr, Mn,         N, and C, respectively.

Since chromium or manganese reacts with carbon, nitrogen, sulfur, and the like to form carbonitrides or acts as a composite precipitate source of these precipitates to serve to improve processability and act as a hydrogen storage source, and thus, since it is necessary to consider the reactivity with carbon and nitrogen complexly as well as each element, the value of Equation 2 may be defined. When the value of Equation 2 is too small, the amount of solid solution element remaining in the steel is increased, which may be the cause of processability deterioration. When the value of Equation 2 is too large, rolling and annealing threading are deteriorated and manufacturing costs may be increased. Therefore, the value of Equation 2 may be 0.0320 to 0.0910. More specifically, it may be 0.033 to 0.089.

In addition to the above components, the present invention includes Fe and inevitable impurities. However, the addition of an effective component other than the components is not excluded. Inevitable impurities may include Ti, Nb, Ni, V, Mo, and the like. In an exemplary embodiment of the present invention, Ti, Nb, Ni, V, Mo, and the like are not added intentionally, and one or more of 0.001 wt % or less of Ti, 0.001 wt % or less of Nb, 0.02 wt % or less of Ni, 0.001 wt % or less of V, and 0.02 wt % or less of Mo may be further included.

FIG. 1 shows a schematic view of the cross-section of the steel sheet for porcelain enamel according to an exemplary embodiment of the present invention. As shown in FIG. 1 , an oxide layer 20 is included in a direction from the surface to the inside of the surface of a steel sheet. The oxide layer 20 includes 5 wt % or more of oxygen (O) and is distinguished from the steel sheet substrate 10 including less than 5 wt % of oxygen (O). Specifically, regarding the cross-section of the steel sheet, when analyzing an oxygen concentration in a direction from the surface to the inside, the oxide layer 20 and a substrate 10 are distinguished based on a point including 5 wt % of oxygen. When there are a plurality of points including 5 wt % of oxygen, the innermost point is identified as a starting point.

The oxide layer 20 may include 90 wt % or more of an Fe oxide.

Since an enamel product is a product in which a glaze which is an organic material is attached on a steel sheet, it is very important to secure adhesiveness between a steel sheet and a glaze. In general, the main component of a glaze is composed of a silicon-oxide SiO₂ system and in order to prevent deterioration of adhesiveness to a steel sheet, an expensive glaze to which a large amount of NiO and the like among the glaze components are added is often applied.

In an exemplary embodiment of the present invention, a method capable of improving enamel adhesiveness by controlling the thickness of the oxide layer on the surface of the steel sheet was confirmed by repetitive experiments. The thickness of the oxide layer mainly composed of a FeO system is managed to a certain range, thereby promoting a covalent bond to a silicon (Si) atom of a glaze layer to improve enamel adhesiveness, and to this end, the thickness of the oxide layer needs to be managed to 0.006 to 0.030 μm. When the thickness of the oxide layer is too small, a bonding force between a glaze layer and a steel sheet is deteriorated, so that it is difficult to secure enamel adhesiveness, but when the thickness of the oxide layer is too thick, it may be advantageous in terms of adhesiveness, but the surface properties of a steel sheet are deteriorated. Therefore, the oxide layer 20 may have a thickness of 0.006 to 0.030 μm. More specifically, the oxide layer 20 may have a thickness of 0.007 to 0.028 μm. The thickness of the oxide layer 20 may be different throughout the steel sheet 100, and in an exemplary embodiment of the present invention, the thickness of the oxide layer 20 refers to an average thickness to the entire steel sheet 100.

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may satisfy the following Equation 3:

0.45≤(R _(a)×50×S _(e))/P _(c)≤0.99  [Equation 3]

-   -   wherein P_(c) is the number of surface irregularities per unit         centimeter (cm), R_(a) is an average roughness value (μm), and         S_(e) is a temper reduction rate (%).

When the value of Equation 3 is small, the wedge effect of the surface of the steel sheet may be decreased to deteriorate the adhesiveness with a glaze. However, when the value of Equation 3 is too large, the crystal grains of the steel sheet grow after an enamel firing treatment, so that it may be difficult to secure the target material and enamel characteristics. More specifically, the value of Equation 3 may be 0.4600 to 0.9500.

As described above, the steel sheet for a high-strength porcelain enamel having excellent adhesiveness according to the present invention may have excellent strength characteristics and excellent enamel adhesiveness.

Specifically, the steel sheet for porcelain enamel having excellent adhesiveness according to an example of the present invention may have a yield strength of 220 MPa or more after an enamel firing heat treatment. The yield strength of the material used in a structural member is a physical property which influences dent resistance and shape fixability of a member, and is usually measured by a tensile test method. In the case of an enamel product, a yield strength in the entry side of processing the product which is produced and supplied by steel companies is important, but the product is subjected to a firing heat treatment performed at a high temperature for drying after an enamel glaze treatment due to the nature of the product. Herein, the heat treatment may vary depending on the kind of used glaze, but may be performed at a temperature of 780 to 850° C. for 15 minutes. As such, among the characteristics of the enamel product, the yield strength after the heat treatment in the enamel treatment process is the main factor in the product stability review, and thus, is limited to 220 MPa or more. Since the yield strength which is usually measured by a tensile test method is a characteristic which may slightly change depending on test conditions, a cross head speed of 10 mm/min which shows a tensile speed per unit hour was applied in the present evaluation. The yield strength after the enamel firing heat treatment which is obtained therefrom may be 220 MPa or more, and more specifically 225 MPa or more. Herein, though the upper limit of the yield strength is not particularly limited, it may be, for example, 350 MPa.

The steel sheet for porcelain enamel according to an exemplary embodiment of the present invention may have an enamel adhesiveness of 95% or more. By satisfying the physical properties, it may be applied as a material for porcelain enamel even when a relatively inexpensive glaze is used. When the enamel adhesiveness is too poor, a glaze layer falls off in a distribution or handling process after the enamel treatment, which decreases a commercial value as an enamel material. Thus, in enamel companies, an expensive glaze to which a large amount of components such as NiO are added is applied, considering stability, which acts as a factor of a cost increase, and thus, efforts are being made to prepare a method capable of securing enamel adhesiveness even with an inexpensive glaze. Usually, when the enamel adhesiveness is 90% or more, the product is classified as the best enamel product, but in an exemplary embodiment of the present invention, a method of securing an enamel adhesiveness of 95% or more is suggested. In addition, when the enamel adhesiveness is lowered, a fish scale incidence by hydrogen is increased in the steel, and thus, it is preferred to secure adhesiveness as high as possible, and in the present invention, the enamel adhesiveness of 95% or more is secured in terms of adhesiveness characteristic and fish scale control. More specifically, the enamel adhesiveness may be 96% or more. The enamel adhesiveness refers to a numerical value expressed by indexing a falling off degree of an enamel glaze layer after applying a certain load to an enamel layer with a hard ball and evaluating a current carrying degree in the area, as defined in ASTM C313-78 of American Society for Testing and Materials Standards. Though the upper limit of the enamel adhesiveness is not particularly limited, it may be, for example, 100%.

The steel sheet for porcelain enamel according to an exemplary embodiment may have a hydrogen permeation ratio of 600 sec/mm 2 or more. The hydrogen permeation ratio is a representative index for evaluating fish scale resistance which shows resistance to fish scale defects which are a fatal defect when an enamel steel manufactured using the cold rolled steel sheet according to an exemplary embodiment of the present invention is applied, and evaluates an ability to fix hydrogen in the steel sheet by the method listed in European standards (EN10209). It is a value obtained by measuring a time (t_(s), unit: second) during which hydrogen is produced in one direction of the steel sheet and hydrogen permeates in the opposite side of the steel sheet, and dividing the time by the square of the material thickness (t, unit: mm), and is shown as t_(s)/t² (unit: sec/mm²). When the hydrogen permeation ratio is too low, in the case of evaluating resistance of the fish scale defects by an accelerated heat treatment at 200° C. for 24 hours after the enamel treatment, a defect rate is 50% or more, which is problematic in use as a stable enamel product, and thus, in order to secure a steel sheet having excellent fish scale resistance, the hydrogen permeation ratio needs to be managed to 600 sec/mm 2 or more. In addition, more specifically, the hydrogen permeation ratio may be 610 sec/mm 2 or more. Though the upper limit of the hydrogen permeation ratio is not particularly limited, it may be, for example, 1700 sec/mm².

The method of manufacturing a steel sheet for porcelain enamel according to an exemplary embodiment of the present invention includes: hot rolling a slab including, by weight: 0.0005 to 0.0030% of C, 0.15 to 0.55% of Mn, 0.001 to 0.03% of Si, 0.0001 to 0.002% of Al, 0.001 to 0.020% of P, 0.001 to 0.030% of S, 0.02 to 0.06% of Cu, 0.005 to 0.012% of N, 0.05 to 0.20% of Cr, and 0.03 to 0.06% of O, with a balance of Fe and inevitable impurities to manufacture a hot rolled steel sheet; cold rolling the hot rolled steel sheet to manufacture a cold rolled steel sheet; annealing the cold rolled steel sheet; and temper rolling the annealed cold rolled steel sheet.

First, a slab satisfying the composition described above is prepared. A molten steel of which the components are adjusted to the composition described above in a steelmaking step may be manufactured into a slab by continuous casting. The alloy component of the slab is substantially the same as the steel sheet for porcelain enamel described above. Since the alloy components are as described above, the overlapping description will be omitted.

Before hot rolling the slab, the manufactured slab may be heated. By the heating, a subsequent hot rolling process may be smoothly performed and the slab may be homogenized. More specifically, the heating may refer to reheating. Herein, a slab heating temperature may be 1150 to 1280° C. When a slab heating temperature is too low, a rolling load may be rapidly increased in a subsequent hot rolling process to deteriorate workability. However, when a slab heating temperature is too high, energy costs increase and a surface scale amount increases, which may lead to a material loss. More specifically, the temperature may be 1180 to 1260° C.

Thereafter, the heated slab is hot rolled to manufacture a hot rolled steel sheet.

Herein, the finish rolling temperature of the hot rolling may be 910 to 970° C. When the finish hot rolling temperature is too low, rolling is finished in a low temperature area to rapidly proceed with mixed granulation of crystal grains, resulting in deterioration of rollability and processability. However, when the finish hot rolling temperature is too high, exfoliation of surface scale is deteriorated and uniform hot rolling is not performed throughout the thickness, and thus, impact toughness may be deteriorated by crystal grin growth. More specifically, a finish hot rolling temperature may be 920 to 960° C.

Thereafter, a hot rolled steel sheet manufactured after the hot rolling is subjected to a winding process. More specifically, it may be a hot rolling and winding process.

Herein, a winding temperature may be 580 to 720° C. The hot rolled steel sheet may be cooled on a run-out-table (ROT) before winding. When a hot rolled winding temperature is too low, a temperature non-uniformity in a width direction occurs in a cooling and maintaining process and precipitation production at a low temperature is changed to cause material deviation and adversely affect enamel properties. However, when the winding temperature is too high, clumping of carbides proceeds to deteriorate corrosion resistance and promote boundary segregation of P to deteriorate cold rolling properties and deteriorate processability by structure coarsening in a final product. More specifically, the winding temperature may be 590 to 710° C.

Before cold rolling the wound hot rolled steel sheet, pickling the steel sheet may be further included.

Thereafter, the wound hot rolled steel sheet is manufactured into a cold rolled steel sheet by cold rolling. Herein, a cold reduction rate may be 60 to 90%. When the cold reduction rate is too low, a recrystallization driving force in a subsequent heat treatment process is not secured, and thus, unrecrystallized grains remain locally, so that strength is increased but processability is significantly deteriorated. In addition, since crushability of carbide formed in a hot rolling step is deteriorated, sites to store hydrogen are decreased, so that it is difficult to secure fish scale resistance, and considering the thickness of a final product, the thickness of a hot rolled place should be lowered, which deteriorates rolling workability. However, when the cold reduction rate is too high, the material is hardened to deteriorate processability and increase the load of roller to deteriorate operability. More specifically, a cold reduction rate may be 63 to 88%.

Thereafter, the cold rolled steel sheet may be annealed to manufacture an annealed steel sheet, in which the annealing treatment may refer to a continuous annealing treatment.

A cold rolling material has a high strength but poor processability due to high deformation applied by cold rolling, and thus, processability is secured by carrying out a continuous annealing treatment. In an example of the present invention, an annealing temperature may be 760 to 840° C., and an appropriate maintenance time may be 10 to 90 seconds.

Herein, the annealing temperature may be 760 to 840° C. When the annealing temperature is too low, deformation formed by the cold rolling is not sufficiently removed, and thus, processability may be significantly deteriorated. However, when the heat treatment temperature is too high, annealing threading may be deteriorated, for example, a possibility of plate fracture occurrence is increased by softening by strength deterioration at a high temperature. Therefore, the annealing temperature may be 760 to 840° C. More specifically, the annealing temperature may be 770 to 830° C.

Besides, in the continuous annealing process, the maintenance time may be 10 to 90 seconds. When a cracking time at a maintenance temperature is too short, unrecrystallized grains remain to act as a factor to greatly deteriorate moldability, but when the maintenance time is too long, crystal grain growth occurs to cause material softening. Thus, since it is difficult to secure a material targeted by a firing heat treatment which is a subsequent process, a maintenance time at the annealing temperature may be 10 to 90 seconds. More specifically, it may be 15 to 80 seconds.

Next, after the annealing of a cold rolled steel sheet, temper rolling the heat treated steel sheet. Though the shape of a material may be controlled and a surface roughness to be desired may be obtained by the temper rolling, when the temper reduction rate is too high, the material is hardened by work hardening and a yield strength is rapidly decreased by structure growth during a subsequent firing heat treatment to deteriorate dent resistance, and thus, the temper rolling may be applied at a reduction rate of 0.4 to 2.0%. More specifically, the reduction rate of temper rolling may be 0.5 to 1.8%. Furthermore, in order to dry an enameled glaze after the temper rolling of an annealed steel sheet, enamel firing may be further included. By heating to a high temperature by an enamel firing process, cooling to room temperature, and applying an enamel layer on the surface of the steel sheet, various properties to suit the application, such as chemical resistance and heat resistance of an enamel product may be obtained, but when a firing temperature is too low, the adhesiveness of the enamel layer may not be secured, and when a firing temperature is too high, it acts as a cost increase factor by an increase in an energy source used, and thus, the firing temperature may be applied as 780 to 850° C. More specifically, the firing temperature may be 790 to 840° C.

Hereinafter, the present invention will be described in more detail by the following examples. However, it should be noted that the following examples are only for illustrating the present invention to describe the present invention in more detail, and the scope of the present invention is not limited thereto. The scope of the present invention is determined by the matters described in the claims and the matters which are reasonably inferred therefrom.

EXAMPLES

A slab was manufactured by conversion-secondary refinement-casting processes with alloy components having the composition in the following Table 1 with a balance of iron (Fe) and inevitable impurities. The slab was maintained in a heating furnace at 1230° C. for 3 hours, and then was hot rolled. At this time, the final thickness of the hot rolled steel sheet was worked at 4.0 mm. The hot rolled specimen was pickled to remove an oxide film from the surface, and cold rolling was performed at a reduction rate. The specimen after the cold rolling was processed into an enamel treatment specimen for searching enamel properties and a specimen for analyzing mechanical properties, which were then heat treated. The finish hot rolling temperature, the winding temperature, the cold reduction rate, the annealing temperature, and the maintenance time are summarized in the following Table 2.

The operability, the enamel properties, the structure characteristics, and the like by manufacturing conditions of the material secured by the above process are shown in the following Table 3.

The threading was indicated as “◯” when the operability of 90% or more was shown and indicated as “X” when the productivity was 90% or less or a defect incidence rate was 10% or more, as compared with the productivity of a common material in the hot rolling and cold rolling processes.

The processability was evaluated as to whether crush or stretch strain occurred during stretching processing of an enamel steel, and when the defects did not occur, it was indicated as “◯”, and when cracks and the like occurred, it was indicated as “X”.

The yield strength (MPa) is a result obtained by subjecting a steel sheet to a firing heat treatment at 820° C. for 15 minutes in a firing furnace, in order to simulate the enamel glaze drying process, manufacturing a tensile specimen, and performing a tensile test at a cross head speed of 10 mm/min.

The bubble defects were determined in three steps of “◯” (excellent), “Δ” (normal), and “X” (poor), respectively, after visually observing an enamel surface of the specimen which was maintained in an oven at 200° C. for 24 hours after the enamel firing treatment.

The enamel treated specimen was cut into an appropriate use so that it was fit for the test purpose, the specimen for treating porcelain enamel which was heat treated was completely degreased, a standard glaze (Check frit) which is relatively vulnerable to fish scale defects was applied thereon, and moisture was removed by maintaining the specimen at 300° C. for 10 minutes. The dried specimen was subjected to an annealing treatment at each enamel firing temperature for 15 minutes, in order to highlight the difference of enamel properties such as adhesiveness, and cooled to room temperature, and as the atmosphere conditions of the firing furnace, a harsh condition of a dew point temperature of 20° C. in which fish scale defects relatively easily occurs was selected.

The specimen after the enamel treatment was subjected to a fish scale acceleration experiment in which the specimen was maintained in an oven at 200° C. for 24 hours. After the fish scale acceleration treatment, it was visually observed whether the fish scale defects occurred, and “◯” was indicated when the fish scale defects did not occur, and “X” was indicated when the fish scale occurred.

The enamel adhesiveness which evaluates adhesiveness between a steel sheet and a glaze is shown by indexing a falling off degree of an enamel glaze layer by applying a certain load to an enamel layer with a hard ball and then evaluating a current carrying degree in the area, as defined in ASTM C313-78 of American Society for Testing and Materials Standards. In the results of enamel adhesiveness evaluation in the present invention, the goal was set to secure adhesiveness of 95% or more in terms of securing application stability of a relatively inexpensive glaze.

The hydrogen permeation ratio is one of the indexes to evaluate resistance to a fish scale which is a fatal defect of porcelain enamel, and is a value obtained by measuring a time (t_(s), unit: second) during which hydrogen was produced in one direction of the steel sheet and hydrogen permeated in the opposite side of the steel sheet, and dividing the time by the square of the material thickness (t, unit: mm), by an experimental method indicated in the European standards (EN10209), which is shown as t_(s)/t² (unit: sec/mm²).

TABLE 1 Value of Value of Classification C Mn Si Al P S N Cu O Cr Equation 1 Equation 2 Inventive 0.0014 0.38 0.009 0.0006 0.009 0.009 0.0078 0.036 0.0364 0.108 4.00 0.067 Steel 1 Inventive 0.0018 0.29 0.007 0.0009 0.012 0.014 0.0092 0.055 0.0422 0.153 4.583 0.053 Steel 2 Inventive 0.0015 0.46 0.012 0.0005 0.011 0.011 0.0087 0.054 0.048 0.184 4.909 0.089 Steel 3 Inventive 0.0022 0.31 0.02 0.0006 0.008 0.017 0.0109 0.029 0.0534 0.098 3.625 0.049 Steel 4 Inventive 0.0026 0.24 0.024 0.0010 0.014 0.008 0.0105 0.046 0.0406 0.115 3.286 0.034 Steel 5 Comparative 0.0019 0.07 0.011 0.0380 0.008 0.010 0.0024 0.012 0.0013 0.173 1.50 0.006 Steel 1 Comparative 0.0027 0.68 0.015 0.0008 0.034 0.045 0.0035 0.053 0.0425 0.358 1.559 0.031 Steel 2 Comparative 0.0086 0.36 0.022 0.0005 0.015 0.011 0.0025 — 0.0381 0.91 0 0.007 Steel 3 Comparative 0.0025 0.54 0.011 0.0511 0.008 0.015 0.0099 0.042 0.0015 1.068 5.25 0.127 Steel 4 Comparative 0.0067 0.22 0.218 0.0013 0.067 0.021 0.0144 0.356 0.0151 0.082 5.313 0.016 Steel 5

TABLE 2 Finish hot Cold Temper Steel rolling Winding reduction Annealing Maintenance reduction type temperature temperature rate temperature time rate Value of Classification No. (° C.) (° C.) (%) (° C.) (sec) (%) Equation 3 Inventive Inventive 930 680 75 780 30 1.4 0.8573 Example 1 Steel 1 Inventive Inventive 930 680 80 800 50 1 0.5875 Example 2 Steel 1 Inventive Inventive 930 680 85 820 70 0.6 0.4606 Example 3 Steel 1 Inventive Inventive 925 620 70 790 40 1.5 0.9395 Example 4 Steel 2 Inventive Inventive 925 620 85 810 60 0.8 0.5091 Example 5 Steel 2 Inventive Inventive 950 600 75 780 25 1.2 0.8703 Example 6 Steel 3 Inventive Inventive 940 640 75 800 40 1 0.5733 Example 7 Steel 4 Inventive Inventive 940 640 80 780 40 0.8 0.5667 Example 8 Steel 5 Inventive Inventive 940 640 80 820 60 0.8 0.5761 Example 9 Steel 5 Comparative Inventive 750 680 80 650 50 1 0.3582 Example1 Steel 1 Comparative Inventive 925 760 50 790 10 2.2 2.2611 Example2 Steel 2 Comparative Inventive 950 460 75 900 50 1 0.3905 Example3 Steel 3 Comparative Inventive 940 640 93 800 120 0.3 0.1269 Example4 Steel 4 Comparative Comparative 930 640 75 800 40 2.1 3.255 Example5 Steel 1 Comparative Comparative 930 640 75 800 40 0.8 0.3775 Example6 Steel 2 Comparative Comparative 940 640 75 800 40 0.8 0.3138 Example7 Steel 3 Comparative Comparative 940 640 75 800 40 0.8 0.3833 Example8 Steel 4 Comparative Comparative 940 640 75 800 40 0.8 0.3556 Example9 Steel 5

TABLE 3 Oxide Occurrence Occurrence Hydrogen layer Yield of of Enamel permeation thickness Process strength bubble fish adhesiveness ratio Classification Threading (μm) ability (MPa) defects scale (%) (sec/mm²) Inventive ◯ 0.012 ◯ 264 ◯ ◯ 99.9 1026 Example 1 Inventive ◯ 0.018 ◯ 248 ◯ ◯ 100 1249 Example 2 Inventive ◯ 0.021 ◯ 239 ◯ ◯ 99.4 1135 Example 3 Inventive ◯ 0.011 ◯ 249 ◯ ◯ 100 982 Example 4 Inventive ◯ 0.015 ◯ 292 ◯ ◯ 99.9 894 Example 5 Inventive ◯ 0.008 ◯ 281 ◯ ◯ 99.8 1014 Example 6 Inventive ◯ 0.025 ◯ 254 ◯ ◯ 100 826 Example 7 Inventive ◯ 0.019 ◯ 245 ◯ ◯ 99.7 924 Example 8 Inventive ◯ 0.013 ◯ 286 ◯ ◯ 99.2 795 Example 9 Comparative X 0.002 X 217 X X 84.2 526 Example 1 Comparative X 0.004 ◯ 154 X X 74.8 462 Example 2 Comparative X 0.033 X 121 X X 92.6 551 Example 3 Comparative X 0.004 X 264 X X 76.8 582 Example 4 Comparative X 0.003 ◯ 168 Δ X 88.6 326 Example 5 Comparative ◯ 0.003 X 179 X ◯ 64.2 572 Example 6 Comparative X 0.002 X 182 X X 58.4 509 Example 7 Comparative ◯ 0.001 X 209 X X 77.3 274 Example 8 Comparative X 0.003 X 276 X X 52.6 496 Example 9

As confirmed in Tables 1 to 3, Inventive Examples 1 to 9 which all satisfied the component composition, the manufacturing conditions, and the oxide layer thickness of the present invention had good threading, satisfied the limited ranges of the present invention of the composition ratio and the relational indexes, did not cause enamel defects such as fish scale and bubble defects even under harsh treatment conditions, and satisfied a hydrogen permeation ratio of 600 sec/mm 2 or more, an enamel adhesiveness index of 95% or more, and the yield strength after the enamel firing heat treatment of 220 MPa or more, thereby securing the target characteristics of the present invention.

However, Comparative Examples 5 to 9 which did not satisfy the chemical composition and the composition ratio suggested in the present invention did not satisfy the thickness of the surface oxide layer, the threading, the processability, the hydrogen permeation ratio, the enamel adhesiveness, the yield strength, and the like, and also in most cases, had fish scale defects even by visual observation after the enamel treatment, and thus, had a problem in applicability.

Besides, when the chemical composition and the composition ratio suggested in the present invention were satisfied, but the annealing temperature was too low (Comparative Example 1), the annealing time was too short (Comparative Example 2), the annealing temperature was too high (Comparative Example 3), the annealing time was too long (Comparative Example 4), it was confirmed that the oxide layer thickness was too small or too large, the enamel adhesives was less than 95%, or enamel defects such as bubble defects or fish scale occurred after the enamel treatment, and the threading was not good, and in some cases, the yield strength was less than 220 MPa after the enamel firing treatment, and thus, the target characteristics were not secured as a whole.

FIG. 2 shows the results of analyzing the component distribution in the thickness direction of the cold rolled steel sheet for porcelain enamel according to Inventive Example 5 by GDS. It was confirmed that the innermost point at which the oxygen content was 5 wt % was 0.015 μm, and the oxide layer 20 having a thickness of 0.015 μm was present on the surface.

The present invention is not limited to the exemplary embodiments, but may be produced in various forms different from each other. A person with ordinary skill in the art to which the present invention pertains will understand that the present invention may be carried out in other specific forms without changing the spirit or the essential feature of the present invention. Therefore, the exemplary embodiments described above should be understood to be illustrative in all respects, and not to be restrictive.

[Description of Symbols] 100: Steel sheet for porcelain enamel, 10: Steel sheet substrate, 20: Oxide layer 

1. A steel sheet for porcelain enamel comprising, by weight: 0.0005 to of C, 0.15 to 0.55% of Mn, 0.001 to 0.03% of Si, 0.0001 to 0.002% of Al, 0.001 to 0.02% of P, 0.001 to 0.030% of S, 0.02 to 0.06% of Cu, 0.005 to 0.012% of N, 0.05 to 0.20% of Cr, and 0.03 to 0.06% of O, with a balance of Fe and inevitable impurities, wherein the steel sheet includes an oxide layer in a direction from a surface to an inside, and the oxide layer has a thickness of 0.006 to 0.030 μm.
 2. The steel sheet for porcelain enamel of claim 1, wherein: the following equation 1 is satisfied: 3.05≤[Cu]/[P]≤5.10  [Equation 1] wherein [Cu] and [P] are contents (wt %) of Cu and P, respectively.
 3. The steel sheet for porcelain enamel of claim 1, wherein: the following Equation 2 is satisfied: 0.032≤([Cr]/52+[Mn]/32)×([N]/14)/([C]/12)≤0.091  [Equation 2] wherein [Cr], [Mn], [N], and [C] are contents (wt %) of Cr, Mn, N, and C, respectively.
 4. The steel sheet for porcelain enamel of claim 1, further comprising: one or more of 0.001 wt % or less of Ti, 0.001 wt % or less of Nb, 0.02 wt % or less of Ni, 0.001 wt % or less of V, and 0.02 wt % or less of Mo.
 5. The steel sheet for porcelain enamel of claim 1, wherein: the following Equation 3 is satisfied: 0.45≤(R _(a)×50×S _(e))/P _(c)≤0.99  [Equation 3] wherein P_(c) is the number of surface irregularities per unit centimeter (cm), R_(a) is an average roughness value (μm), and S_(e) is a temper reduction rate (%).
 6. The steel sheet for porcelain enamel of claim 1, wherein: a yield strength after an enamel firing heat treatment is 220 MPa or more.
 7. The steel sheet for porcelain enamel of claim 1, wherein: an enamel adhesiveness is 95% or more.
 8. The steel sheet for porcelain enamel of claim 1, wherein: a hydrogen permeation ratio is 600 sec/mm 2 or more.
 9. A method of manufacturing a steel sheet for porcelain enamel, the method comprising: hot rolling a slab including, by weight: 0.0005 to 0.0030% of C, 0.15 to 0.55% of Mn, 0.001 to 0.03% of Si, 0.0001 to 0.002% of Al, 0.001 to 0.020% of P, 0.001 to 0.030% of S, 0.02 to 0.06% of Cu, 0.005 to 0.012% of N, 0.05 to 0.20% of Cr, and 0.03 to 0.06% of O, with a balance of Fe and inevitable impurities to manufacture a hot rolled steel sheet; cold rolling the hot rolled steel sheet to manufacture a cold rolled steel sheet; annealing the cold rolled steel sheet; and temper rolling the annealed cold rolled steel sheet, wherein the annealing is performed at a temperature of 760 to 840° C. for 10 to 90 seconds.
 10. The method of manufacturing a steel sheet for porcelain enamel of claim 9, wherein: in the manufacturing of a hot rolled steel sheet, a finish hot rolling temperature is 910 to 970° C.
 11. The method of manufacturing a steel sheet for porcelain enamel of claim 9, wherein: in the manufacturing of a hot rolled steel sheet, a winding temperature is 580 to 720° C.
 12. The method of manufacturing a steel sheet for porcelain enamel of claim 9, wherein: in the manufacturing of a cold rolled steel sheet, a reduction rate is 60 to 90%.
 13. The method of manufacturing a steel sheet for porcelain enamel of claim 9, wherein: the temper rolling is rolled at a reduction rate 0.4 to 2.0%.
 14. The method of manufacturing a steel sheet for porcelain enamel of claim 9, further comprising: after the temper rolling, enamel firing the temper rolled steel sheet at a temperature of 780 to 850° C. 